It is recalled that during hot-dip coating of a steel strip, the running strip passes into a pan containing the coating metal or metal alloy, maintained in the liquid state. The coating is deposited on the strip which then emerges from the bath, and passes through a device controlling the thickness of the coating and contributing to its solidification, generally formed by nozzles projecting gas onto the surface of the coating. Before its penetration into the bath, the strip is heated up with an annealing oven and then cooled to a temperature close to the temperature of the bath in order to generate optimum adherence conditions between the strip and the coating.
During the crossing of the bath, within the bath, formation of oxides and intermetallic precipitates occurs, essentially based on Zn and Fe in the case of a galvanization bath, containing liquid zinc which will be considered in a preferential manner in the following of the description, without its being an exclusive application of the invention. These precipitates are called “dross”. Certain dross have a higher density than that of the bath, and decant at the bottom of the pan without interfering with the galvanization process. Others, on the other hand, have a lower density than that of the bath and float at its surface. They may be incorporated to the coating of the strip, and therefore may generate defects therein. These low density dross, which will be the only ones considered in the following of the text, should therefore be as far away as possible from the area where the strip enters the bath (if this entrance is carried out in the open air, which is not always the case) and from the area where the strip exits out of the bath, and be removed from the pan gradually as they are formed.
For this purpose, most conventionally, an operator standing near the pan pushes, with a tool, the dross towards a container located away from the entrance and exit areas of the strip, this container then being extracted from the pan and emptied by means of an automated system or not. In other cases, the operator pushes the dross towards an area of the pan where a device such as a robot discharges them towards a container outside the pan, in which they are collected.
This operation is uncomfortable and potentially dangerous for the operator, since he/she has to stand in close proximity to a bath of hot liquid metal, with the inconveniences and risks related to the heat and to the possibility of projections of the liquid metal. Moreover the system for controlling the coating thickness deposited on the strip, consists of blowing nozzles, and may use inert gases such as nitrogen in order to limit oxidation of the coating. The use of these inert gases is also a source of risks for the operator, because of the lack of oxygen in the atmosphere around the pan which this involves.
Further, this operation for cleaning the dross imposes a limit on the running speed of the strip, since high speed promotes production of dross, for which the operator and the robot should have time for their removal.
Also, the higher the speed of the strip, the more the nozzles for controlling the coating thickness have to project a substantial amount of gas for maintaining the coating thickness constant. This has the effect of increasing the ambient temperature around the bath, since the blowing gas conveys heat from the strip and the bath towards the working area of the operators.
Finally, in order to limit the thermal energy losses related to the heating of the bath, it is contemplated that certain new coating facilities are entirely encased. It would therefore be necessary, in this case, to limit external interventions and notably that of an operator for removing dross, in order to avoid too frequent removals of covers of the facility.
There is therefore a need for increasing the safety, the rapidity and the efficiency for removing the dross as compared with this conventional technique, without however radically changing the actual galvanization method and the general design of the facility which applies it.
A solution devised by certain steelmakers has been to at least essentially replace human intervention for bringing the dross into the action area of the robot, with the action of electromagnetic devices. By means of sliding fields generated by inductors such as linear motors, electromagnetic forces, to which the metal or liquid metal alloy are sensitive (so-called “magnetomotive forces”), causes displacement of the metal or liquid metal alloy which carries away the dross into the area of the pan where the robot is active, by generating a dross recirculation path for leading them into said area. Such devices are for example described in documents JP-A-10-053850, JP-A-54-33234, JP-A-2005-068545, JP-11-006046.
JP-A-54-33234, for example, teaches that inductors with a sliding field should be positioned all around the strip in its area for exiting the pan, the sliding fields bringing the dross into the corner of the pan where a conveyor belt is found, which removes the dross out of the pan into a container which collects them. In its case, entering the strip into the galvanization bath is performed, as this is often the case, inside a tube immersed in the bath and connected upstream to the annealing oven, and the dross which have decanted at the surface of the bath, cannot come into contact with the surface of the strip in this area. It is therefore sufficient to place inductors in the surroundings of the area where the strip exits.
JP-A-10-053850 teaches that screens should be positioned parallel to the strip in its entrance area in the pan, and inductors with a sliding field should be positioned in the vicinity of the two ends of each screen. The thereby generated magnetic fields give the possibility of attracting the dross out of the area comprised between the screens and including the strip.
In the case when there is no robot, such devices anyway make the work of the operator easier, who has only to act in the area of the pan, the surface of which is relatively limited.